Thermal Wind Balance Questions and Answers

Answer: C

The 1000 to 500 hectopascal thickness spans roughly the lower half of the atmosphere by mass and is the traditional field for identifying air masses and forecasting precipitation type. Its typical mid-latitude value is around 540 to 570 decametres. The other layers are used for more specialised purposes, such as identifying warm noses aloft or examining the upper level thermal structure near the jet.

Answer: D

The 540 decametre contour of the 1000 to 500 hectopascal thickness is the classical rule of thumb for the rain and snow boundary at low levels. It is only a guide, because the distribution of temperature within the layer matters as much as its mean, and a shallow warm layer can melt snow that a bulk value would predict. The 528 value is genuinely used as a stricter threshold, which is why it makes a plausible distractor. Checking the units of each quantity catches a wrong formula before the arithmetic begins.

Answer: A

Thickness is a mass-weighted mean, so it conceals the vertical distribution of temperature, and a warm nose only a few hundred metres deep is enough to turn snow into rain or freezing rain. Examining the full profile is therefore always preferable. Thickness is computable anywhere the analysis extends, and while the rule is used mainly in winter that is a matter of relevance rather than validity.

Answer: B

Since the thermal wind is by definition the difference between the two geostrophic winds, adding it as a vector to the lower one recovers the upper one. Drawing this triangle on a chart was the standard method of estimating upper winds before routine upper air observation, and it remains a useful check. The other constructions have no basis in the relation and would give wrong answers in both magnitude and direction. Naming the cost of a choice alongside its benefit is what turns a fact into a design decision.

Answer: C

If the wind veers with height, the thermal wind lies to the right of the lower wind, and since the thermal wind has cold air on its left the flow must be carrying warm air into the layer. This single observation from a sounding gives the sign of the thermal advection with no need for any temperature data at all. In the southern hemisphere the rule reverses, which is a point often omitted when the mnemonic is quoted. Working one numerical case through both cases side by side fixes the distinction better than any rule of thumb.

Answer: D

Backing places the thermal wind to the left of the lower level wind, and with cold air on the left of the thermal wind the flow must be advecting cold air into the layer. This typically appears behind a cold front and is often accompanied by falling thickness. Humidity is not determined by the turning, and a purely directional change without any advection would require the isotherms to be parallel to the flow, which is a different situation entirely.

Answer: A

Advection is the wind dotted with the temperature gradient, so it is greatest when the flow crosses the isotherms at right angles and vanishes when the flow is along them. Speed matters too, but orientation is what distinguishes strong advection from none. A wind that only changes speed with height implies a thermal wind parallel to the flow, which means the isotherms are parallel to the flow and the advection is zero. A single measurement chosen to respond to the one asymmetry beats several that respond to shared behaviour.

Answer: B

When the thermal wind is parallel to the flow, the isotherms are also parallel to the flow, so no temperature is being carried across them. The wind then changes speed but not direction with height. A barotropic layer has no thermal wind at all, which is a different case, and equal speeds at top and bottom would indicate a thermal wind perpendicular to the mean flow and hence maximum advection.

Answer: C

In a baroclinic atmosphere the isobaric and isopycnal surfaces intersect, so the solenoidal term is non-zero and circulation can be generated. Equivalently, there is a horizontal temperature gradient on a pressure surface and hence a thermal wind. A barotropic atmosphere is the opposite case with no shear. Isentropic and adiabatic describe constant potential temperature and the absence of heat exchange respectively, which are different ideas.

Answer: D

With no horizontal temperature gradient the thermal wind vanishes, so the geostrophic wind is identical at every level. This is why the earliest numerical forecast models, which were barotropic, could represent the flow with a single level. The other options describe baroclinic situations with a thermal wind oriented parallel, antiparallel or perpendicular to the flow respectively.